Metal extractant reagents having increased resistance to degradation

ABSTRACT

Solvent extraction compositions having an orthohydroxyaryloxime extractant, an anti-degradation agent, and a water-immiscible organic solvent, processes for extracting a metal from an aqueous acidic solution using same, and methods of reducing degradation of such compositions are provided herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/575,580, filed Oct. 8, 2009 (allowed), which claims benefit ofpriority to U.S. Provisional Application No. 61/105,252, filed Oct. 14,2008 the contents of each of which is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates to compositions of organic solubleanti-degradation agents and oxime extractants for use in the solventextraction of metals from nitrate containing feeds. More specifically,the compositions in accordance with the invention are resistant tonitration and hydrolysis when used in the presence of these nitratecontaining feeds.

BACKGROUND OF THE INVENTION

Solvent extractants are well known for the recovery of metals,especially copper, from aqueous streams and include oxime reagents,particularly o-hydroxyarylaldoximes and o-hydroxyarlyketoximes. Whilesuch reagents have been found to work well in the recovery of copperfrom solution, one problem that has been encountered in the applicationof such reagents is that the aldoxime and ketoxime reagents which are incontinual use in metal extraction circuits can be degraded and the levelof degradation can impair the efficiency of metal transfer from leachsolution to strip solution. In particular it has been noted that thisproblem is more pronounced when metal values are extracted from certainores. In Chile there are large mineral deposits of what are referred toas porphyry copper deposits. Normally such deposits do not contain highnitrate concentrations, but in very few parts of the world the depositsmay contain large amounts of nitrate. Of particular note are the copperdeposits in the Atacama Desert region of northern Chile, wheremineralogy of the region gives rise to nitratine or Chile saltpetre(NaNO₃), a mineral with high solubility as well as nitre (KNO₃). Theseores have been found to cause problems in the solvent extraction processespecially with the degradation of solvent extraction reagents.

In recent years a number of articles and patents have appeared whichhave offered ways to reduce the rate of degradation of oximes in contactwith nitrate containing feeds. Many of these involved operationaladjustments to the pregnant leach solutions (PLS). These includedreducing the nitrate concentration, raising the pH, controlling theredox potential of the feed, and reducing the transfer of impuritiesfrom the leach solution to the electrolyte. Given the high flow rates ofthe PLS in the solvent extraction processes, combining one or more ofthese adjustments into a extraction circuit flow sheet is not a trivialmatter. Addition of nitrous acid quenchers such as sulphamic acid orurea to the circulating electrolyte is shown to be an effectiveprotection for the oximes if nitrate ions are transferred from the leachto electrolyte solution. Another way to address the problem was to use areagent based on 2-hydroxy-5-alkyl-acetophenone oxime (ketoxime) whichhas been reported to show increased stability to that ofsalicylaldoximes in contact with aqueous streams of this type. Ketoximeformulations do contain a higher concentration of residual nonyl phenolcarried through from their manufacture and, as shown in this applicationfor patent, this residual phenol could, in part, be responsible fortheir increased stability over that of salicylaldoximes. Ketoximes areweaker reagents than salicylaldoximes so in using theseo-hydroxyarylketoximes the copper extraction could be limited whenmoving to higher tenor or lower pH feeds. Providing a strong oximeformulation that is stable to these aggressive aqueous feeds wouldnegate the need for costly adjustments to the PLS whilst enabling themaximium copper recovery from high copper/low pH solutions.

Reagent formulations which could resist this enhanced degradation incontact with these streams or similar conditions would be advancement onexisting technology.

SUMMARY OF THE INVENTION

The use of organic soluble anti-degradation agents in formulation withmodified or unmodified salicylaldoxime and/or ketoxime extractants toreduce the rate of nitration and hydrolysis of the oximes during boththe extraction and stripping of copper from the organic phase whencontacted with nitrate containing streams.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

The present invention concerns a solvent extraction composition, asolvent extraction process and especially a process for the extractionof metals, particularly copper from aqueous solutions, especiallysolutions obtained by leaching ores.

It is known to extract metals, especially copper from aqueous solutionscontaining the metals in the form of, for example, salts, by contactingthe aqueous solution with a solution of a solvent extractant in a waterimmiscible organic solvent and then separating the solvent phase loadedwith the metals, i.e. containing at least a part of the metals in theform of a complex. The metals can then be recovered by stripping with asolution of lower pH (the electrolyte) followed for example, byelectrowinning. Most commonly, the aqueous metal-containing solutionsfor extraction are the result of the acid leaching of ores.

Solvent extractants which have found favour in recent years particularlyfor the recovery of copper from aqueous solutions include oximereagents, especially o-hydroxyarylaldoximes and o-hydroxyarylketoximes.While such reagents have been found to work well in the recovery ofcopper from solutions, one problem that has been encountered in theapplication of such reagents is that the aldoxime and ketoxime reagentswhich are in continual use can be degraded and the level of degradationcan be such that the efficiency of metal transfer from leach solution tostrip solution can be impaired. In particular it has been noted thatthis problem is more pronounced when metal values are extracted fromcertain ores. In Chile there are large deposits of what are referred toas porphyry copper deposits. Normally deposits do not contain highnitrate concentrations, but in very few parts of the world the depositsmay contain large amounts of nitrate. Of particular note are the copperdeposits in the Atacama Desert region of northern Chile, wheremineralogy of the region gives rise to nitratine or Chile saltpetre(NaNO3), a mineral with high solubility as well as nitre (KNO3). Theseores have been found to cause problems in the solvent extraction processespecially with the degradation of solvent extraction reagents. Reagentformulations which could resist this enhanced degradation when incontact with these streams or similar conditions would be an advancementon existing technology.

According to a first aspect of the present invention, there is provideda solvent extraction composition comprising one or moreorthohydroxyarylaldoximes and/or one or more orthohydroxyarylketoximes,and/or one or more equilibrium modifiers and one or moreanti-degradation agent present (as a percentage of the in-useformulation) from about 0.1-20%, preferably 0.5-10%. The compositionspreferably also comprise a water immiscible organic solvent.

Compositions according to the present invention may facilitate reducedoxime degradation in solvent extraction circuits in contact with metalaqueous feeds containing nitrate ions.

The orthohydroxyarylketoxime compounds employed in the present inventionare substantially water insoluble and preferably have the formula:

wherein

-   -   R¹ is an optionally substituted hydrocarbyl group    -   R² is an optionally substituted ortho-hydroxyaryl group, and        salts thereof.

The orthohydroxyarylaldoxime compounds employed in the present inventionare substantially water insoluble and preferably have the formula:

wherein

-   -   R³ is an optionally substituted ortho-hydroxyaryl group, and        salts thereof.

While the invention is described herein with reference to compounds ofFormula (1) and (2), it is understood that it relates to said compoundin any possible tautomeric forms, and also the complexes formed betweenorthohydroxyarylaldoximes or orthohydroxyarylketoximes and metals,particularly copper.

Optionally substituted hydrocarbyl groups which may be represented by R¹preferably comprise optionally substituted alkyl and aryl groupsincluding combinations of these, such as optionally substituted aralkyland alkaryl groups.

Examples of optionally substituted alkyl groups which may be representedby R¹ include groups in which the alkyl moieties can contain from 1 to20, especially from 1 to 4, carbon atoms. A preferredorthohydroxyarylketoxime is one in which R¹ is alkyl, preferablycontaining up to 20, and especially up to 10, and more preferably up to3 saturated aliphatic carbon atoms, and most preferably R¹ is a methylgroup.

Examples of optionally substituted aryl groups which may be representedby R¹ include optionally substituted phenyl groups. When R¹ is an arylgroup, it is preferably an unsubstituted phenyl group.

The orthohydroxyarylaldoximes and orthohydroxyarylketoximes are oftenpresent in a total amount of up to 70% by weight of the composition,commonly no more than 60%, and usually no more than 50% w/w. Often, thetotal amount of orthohydroxyarylaldoxime and orthohydroxyarylketoxime inuse comprises at least 1% by weight, commonly at least 2.5% by weightand usually at least 5% by weight of composition, and preferablycomprises from 7.5 to 20%, such as about 10%, by weight of thecomposition.

Equilibrium modifiers employed in the present invention aresubstantially water insoluble. Suitable equilibrium modifiers can bealkylphenols, alcohols, esters, ethers and polyethers, carbonates,ketones, nitriles, amides, carbamates, sulphoxides, and salts of aminesand quaternary ammonium compounds and mixtures thereof.

Anti-degradation agents useful in the present invention should have nodetrimental effect on the copper solvent extraction process. Morespecifically, the anti-degradation agent should not interfere withcopper transfer; it should be selective over other metals likely to bepresent in significant concentration in the leach solution; it shouldnot have a detrimental effect on kinetic performance; it should not havea detrimental effect on stability of the extractant, and it should notbe detrimental to the physical performance of the organic phase. Theanti-degradation agents employed in the present invention aresubstantially water insoluble.

Examples of suitable chemistries include all phenols containing one ormore OH groups, resorcinols catechols, naphthols and mixtures thereof.The ortho and/or para position relative to the hydroxy group should befree from substitution to allow nitration. Compounds should include asolubilising group to ensure diluent solubility (e.g. optionallysubstituted hydrocarbyl groups which may preferably comprise optionallysubstituted alkyl and aryl groups including combinations of these, suchas optionally substituted aralkyl and alkaryl groups having from 6 toabout 20 carbons).

Organic solvents which may be present in the composition include anymobile organic solvent, or mixture of solvents, which is immiscible withwater and is inert under the extraction conditions to the othermaterials present. Preferably the organic solvent has a low aromatichydrocarbon content.

Preferred organic solvents are hydrocarbon solvents which includealiphatic, alicyclic and aromatic hydrocarbons and mixtures thereof aswell as chlorinated hydrocarbons such as trichloroethylene,perchloroethylene, trichloroethane and chloroform.

Highly preferred organic solvents having a low aromatic content includesolvents and solvent mixtures where the amount of aromatic hydrocarbonspresent in the organic solvent is less than 30%, usually less than 23%,often less than 5%, and frequently less than 1%.

Examples of suitable hydrocarbon solvents include ESCAID™ 110, ESCAID™115, ESCAID™ 120, ESCAID™ 200, and ESCAID™ 300 commercially availablefrom Exxon, Houston Tex., SHELLSOL™ D70 and D80 300 commerciallyavailable from Shell Oil, Houston, Tex., and CONOCO™ 170 commerciallyavailable from Conoco, Ogden, Utah. Suitable solvents are hydrocarbonsolvents that include high flash point solvents and solvents with a higharomatic content such as SOLVESSO™ 150 commercially available fromExxon.

More preferred are solvents with a low aromatic content. Certainsuitable solvents with a low aromatic content, have aromatic contents of<1% w/w, for example, hydrocarbon solvents such as ESCAID™ 110commercially available from Exxon, and ORFOM™ SX 10 and ORFOM™ SX11commercially available from Phillips Petroleum, Bartlesville, Okla.Especially preferred, however, on grounds of low toxicity and wideavailability, are hydrocarbon solvents of relatively low aromaticcontent such as kerosene, for example ESCAID™ 100 which is a petroleumdistillate with a total aromatic content of 23% commercially availablefrom Exxon, or ORFOM™ 5X7, commercially available from PhillipsPetroleum.

In many embodiments, the composition comprises at least 30%, often atleast 45% by weight, preferably from 50 to 95% w/w of water-immisciblehydrocarbon solvent.

Advantageously, it may be preferred to make and supply the compositionin the form of a concentrate. The concentrate may then be diluted by theaddition of organic solvents as described herein above to producecompositions in the ranges as described herein above. Where theconcentrate contains a solvent, it is preferred that the same solvent isused to dilute the concentrate to the “in use” concentration range. Inmany embodiments, the concentrate composition comprises up to 30%, oftenup to 20% by weight, preferably up to 10% w/w of water-immisciblehydrocarbon solvent. Often the concentrate composition comprises greaterthan 5% w/w of water-immiscible hydrocarbon solvent. In certain highstrength concentrates it may be necessary to employ a higher than normalaromatic hydrocarbon content. In such cases where a high aromatichydrocarbon containing solvent is used in the concentrate, solvent ofvery low aromatic hydrocarbon content may be used to dilute theconcentrate to the “in use” concentration range.

According to a second aspect of the present invention, there is provideda process for the extraction of a metal from solution in which an acidicsolution containing a dissolved metal is contacted with a solventextraction composition, whereby at least a fraction of the metal isextracted into the organic solution, characterised in that the solventextraction composition comprises a water immiscible organic solvent, oneor more modified or unmodified orthohydroxyarylaldoximes and one or moremodified or unmodified orthohydroxyarylketoximes, and ananti-degradation agent present (as a percentage of the in-useformulation) from about 0.1-20% but preferably 0.5-10%.

Metals that may be extracted in the process according to the secondaspect of the present invention include copper, iron, cobalt, nickel,manganese and zinc, most preferably copper.

The orthohydroxyarylaldoximes, orthohydroxyarylketoximes, theequilibrium modifiers, the anti-degradation agent and the waterimmiscible organic solvent are as herein described before.

The aqueous acidic solution from which metals are extracted by theprocess of the second aspect of the present invention often has a pH inthe range of from −1 to 7, preferably from 0 to 5, and most preferablyfrom 0.25 to 3.5. The solution can be derived from the leaching of oresor may be obtained from other sources, for example metal containingwaste streams.

The concentration of metal, particularly copper, in the aqueous acidicsolution will vary widely depending for example on the source of thesolution. Where the solution is derived from the leaching of ores, themetal concentration is often up to 75 g/l and most often from 1 to 40g/l.

Preferred solvent extraction compositions are those which comprise oneof the following:

1) A blend of 5-(C₈ to C₁₄ alkyl)-2-hydroxybenzaldoxime and 5-(C₈ to C₁₄alkyl)-2-hydroxyacetophenone oxime in a weight ratio of from about 90:10to about 50:50 aldoxime to ketoxime, and/or one or more modifiersselected from 2,2,4-trimethyl-1,3-pentanediol mono-isobutyrate,2,2,4-trimethyl-1,3-pentanediol mono-benzoate,2,2,4-trimethyl-1,3-pentanediol di-isobutyrate,2,2,4-trimethyl-1,3-pentanediol di-benzoate, butyl adipate, pentyladipate, hexyl adipate, isobutyl heptyl ketone, nonanone, diundecylketone, 5,8-diethyldodecane-6,7-dione, tridecanol, and nonyl phenol, andan anti-degradation agent selected from those set forth herein above,present (as a percentage of the in-use formulation) from about 0.1-20%but preferably 0.5-10%.

2) A blend of 5-(C₈ to C₁₄ alkyl)-2-hydroxybenzaldoxime or 5-(C₈ to C₁₄alkyl)-2-hydroxyacetophenone oxime, one or more modifiers selected from2,2,4-trimethyl-1,3-pentanediol mono-isobutyrate,2,2,4-trimethyl-1,3-pentanediol mono-benzoate, 2,2,4-trimethyl-31,3-pentanediol di-isobutyrate, 2,2,4-trimethyl-1,3-pentanedioldi-benzoate, butyl adipate, pentyl adipate, hexyl adipate, isobutylheptyl ketone, nonanone, diundecyl ketone,5,8-diethyldodecane-6,7-dione, tridecanol, and nonyl phenol, and ananti-degradation agent selected from those set forth herein above,present (as a percentage of the in-use formulation) from about 0.1-20%,but preferably from about 0.5-10%.

The process of the second aspect of the present invention can be carriedout by contacting the solvent extractant composition with the aqueousacidic solution. Ambient or elevated temperatures, such as up to 75° C.can be employed if desired. Often a temperature in the range of from 5to 60° C., and preferably from 15 to 40° C., is employed. The aqueoussolution and the solvent extractant are usually agitated together tomaximize the interfacial areas between the two solutions. The volumeratio of solvent extractant to aqueous solution are commonly in therange of from 20:1 to 1:20, and preferably in the range of from 5:1 to1:5. In many embodiments, to reduce plant size and to maximize the useof solvent extractant, organic to aqueous volume ratios close to 1:1 aremaintained by recycle of one of the streams.

After contact with the aqueous acidic solution, the metal can berecovered from the solvent extractant by contact with an aqueous acidicstrip solution.

The aqueous strip solution employed in the process according to thesecond aspect of the present invention is usually acidic, commonlyhaving a pH of 2 or less, and preferably a pH of 1 or less, for example,a pH in the range of from −1 to 0.5. The strip solution commonlycomprises a mineral acid, particularly sulphuric acid, nitric acid orhydrochloric acid. In many embodiments, acid concentrations,particularly for sulphuric acid, in the range of from 130 to 200 g/l andpreferably from 150 to 180 g/l are employed. When the extracted metal iscopper, preferred strip solutions comprise stripped or spent electrolytefrom a copper electro-winning cell, typically comprising up to 80 g/lcopper, often greater than 20 g/l copper and preferably from 30 to 70g/l copper, and up to 220 g/l sulphuric acid, often greater than 120 g/lsulphuric acid, and preferably from 150 to 180 g/l sulphuric acid.

The volume ratio of organic solution to aqueous strip solution in theprocess of the second aspect of the present invention is commonlyselected to be such so as to achieve transfer, per litre of stripsolution, of up to 50 g/l of metal, especially copper into the stripsolution from the organic solution. In many industrial copperelectrowinning processes transfer is often from 10 g/l to 35 g/l, andpreferably from 15 to 20 g/l of copper per litre of strip solution istransferred from the organic solution. Volume ratios of organic solutionto aqueous solution of from 1:2 to 15:1 and preferably from 1:1 to 10:1,especially less than 6:1 are commonly employed.

Both the separation and stripping process can be carried out by aconventional batch extraction technique or column contactors or by acontinuous mixer settler technique. The latter technique is generallypreferred as it recycles the stripped organic phase in a continuousmanner, thus allowing the one volume of organic reagent to be repeatedlyused for metal recovery.

A preferred embodiment of the second aspect of the present inventioncomprises a process for the extraction of a metal from aqueous acidicsolution in which:

in step 1, the solvent extraction composition comprising a waterimmiscible organic solvent, one or more orthohydroxyarylaldoximes and/orone or more orthohydroxyarylketoximes, and one or more equilibriummodifiers and an anti-degradation agent is first contacted with theaqueous acidic solution containing metal,

in step 2, separating the solvent extraction composition containingmetal-solvent extractant complex from the aqueous acidic solution;

in step 3, contacting the solvent extraction composition containingmetal-solvent extractant complex with an aqueous acidic strip solutionto effect the stripping of the metal from the water immiscible phase;

in step 4, separating the metal-depleted solvent extraction compositionfrom the loaded aqueous strip solution.

The invention will be illustrated but not limited by the followingexamples.

EXAMPLES

A screen to evaluate whether anti-degradation additives are effective inscavenging nitrating species in two phase systems is designed. A surfaceactive phenol (p-tertamylphenol, TAP) is incorporated into an organicphase and then this organic phase is contacted with a pregnant leachsolution (PLS) containing various cations and anions (includingnitrate), the level of nitrated phenol measured after contact gives a“nitration potential” for a given PLS composition. In addition tomeasuring the nitration potential for a pregnant leach solution, thisscreen (TAP test) can be used as a measure to indicate nitrationpotential in minesite PLS feeds.

Addition of a water immiscible anti-degradation agent to the organicphase in addition to the TAP followed by contacting the organic phasewith a nitrate containing PLS, gives an indication of the ability of theagent to scavenge nitrating species in competition to the highlyreactive TAP. Anti-degradation agents with the ability to scavengenitrating species under these conditions should protect the lessreactive salicylaldoximes/ketoximes from nitration under theseconditions.

TAP Screen

A solution of TAP (0.0002M) with/without anti-degradation agent (0.004M)in heptane (5 ml) is stirred in contact with a nitrate containing PLS(50 ml) for 3 hours at 35° C. The organic phase is separated and thelevel of nitrated TAP determined by GC. All tests are run in duplicate.

PLS composition:

Copper 7 g/l Ferric 5 g/l Nitrate 45 g/l Chloride 10 g/l

pH 1.2

TABLE 1 Nitrated TAP Nitrated TAP Test Duplicate Anti-degradation agentArea % Average % Control 1 None 79.05 78.1 2 None 77.2  A 1 2

51.89 50.34 51.1 B 1 2

55.3  51.69 53.5 C 1 2

69.2  68.6  68.9 D 1 2

60.2  61.7  60.9 E 1 2

42.8  42.0  42.4 F 1 2

0   0   0 G 1 2

0   0   0 H 1 2

4.4 5.9 5.1

CONCLUSION

In all cases the presence of the water immiscible anti-degradation agentin the organic phase showed the ability to scavenge nitrating specieswhilst in the presence of the highly reactive tert-amyl phenol.

Protection of Oximes in Contact with High Nitrate Feed

To confirm the protective nature of these anti-degradation agents oncommercial oximes the following experiments are completed. To show theactivity of these compounds an anti-degradation agent showing averageactivity from the TAP screen is chosen to exemplify the effect. Reagentused : mixed isomers of (1-Phenylethyl) phenol, (Table 1, test E).

EXAMPLE 1

An aqueous pregnant leach solution (400 ml) having a pH of 2.0 isprepared containing Cu 6 g/l, ferric iron 3 g/l, chloride 10 g/l andnitrate 60 g/l. A kerosene (ORFOM™ SX11) solution (400 ml) containing amixture of 23.6 g 5-nonylsalicyladloxime and 19 g Kodaflex TXIB is alsoprepared. The above acid leach solution and the kerosene solution areplaced in a reaction flask fitted with stirrer and a condenser. Thetemperature is maintained at 40±2° C. with the use of an externalheating bath. The solutions are mixed at 600 rpm with the continuity ofthe phases being aqueous continuous. Samples of the organic areextracted at various time intervals and as the nitrated oxime will notallow the copper to be stripped from the organic a minimum strip valuefor the organic phase is calculated (Organic phase stripped with 2contacts of a 15% sulfuric acid at a 1:5 O/A ratio). If the degree ofnitration becomes severe, a precipitate forms in the organic phase.

As hydrolytic degradation of the oximes also appears during nitration, amaximum load of the organic phase is also calculated.Results are presented in Table 2

EXAMPLE 2

The process of Example 1 is repeated except that 24.8 g/l2-hydroxy-5-nonylacetophenone oxime is present in the kerosene phase inplace of the salicylaldoxime formulation.

Results are presented in Table 2

EXAMPLE 3

The process of Example 1 is repeated except that 7.38 g/l(1-Phenylethyl) phenol (mixed isomer) is present in the kerosene phasein addition to a mixture of 23.6 g 5-nonylsalicyladloxime and 19 gKodaflex TXIB.

Results are presented in Table 2

EXAMPLE 4

The process of Example 1 is repeated except that 14.76 g/l(1-Phenylethyl) phenol (mixed isomer) is present in the kerosene phasein addition to a mixture of 23.6 g 5-nonylsalicyladloxime and 19 gKodaflex TXIB.

Results are presented in Table 2

TABLE 2 Minimum Strip vaue (Cu ppm) Example 3 Example 4 Example 2Salicylaldoxime Salicylaldoxime Example 1 Hydroxyacetophenoneformulation + formulation + Time at Salicylaldoxime oximeanti-degradation anti-degradation 40° C. formulation (Ketoxime) agent(~1.8%) agent (~3.7%)  0 0 0 0 0 170 Precipitated* 0.48 0.27 0.34 336120 0.32 0.35 502 522 0.43 0.44 619 663 0.32 0.28 Max Load 6.78 6.716.78 6.73 @ 0 hours Max Load 2.70 5.53 6.47 6.49 @ 619 hours *nitrationat such a level that the nitrated copper complex precipitated

Conclusion

Addition of the anti-degradation agent to the salicylaldoximesignificantly increased the stability of the oxime to both nitration andhydrolysis as compared to both the unstabilized salicylaldoximeformulation and the commercially favoured ketoxime.

Application Tests

Apart from stabilizing oximes in contact with nitrate containing feeds,any additive must not produce detrimental effects on the metallurgicalperformance of the commercial reagent. The following tests carried outusing the Acorga Extraction Reagents: Standard Test Methods compared themetallurgical performance of the formulations in examples 1-4, beforeand after the degradation test.

-   -   (i) Extraction kinetics    -   (ii) Cu:Fe selectivity    -   (iii) Phase disengagement

Salicylaldoxime Salicylaldoxime formulation + formulation +Hydroxyacetophenone anti- anti- Salicylaldoxime oxime degradationdegradation Test formulation (Ketoxime) agent (~10%) agent (~20%)Extraction Kinetics (% ATE) At 0 hours 98.6 92.1 99.6 98.8 At 619 hoursprecipitate 85.9 91.1 96.2 Cu:Fe Selectivity At 0 hours 4990:1 2237:13831:1 3414:1 At 619 hours precipitate 1023:1 1653:1 1865:1 Phasedisengagement (seconds) (organic cont) At 0 hours 27 116 27 34 At 619hours precipitate 3039 139 229

Conclusion

It can be seen that the metallurgical properties of the commercialformulations not containing the anti-degradation agent are significantlyaffected compared with that of the formulations containing theanti-degradation agent.

What is claimed is:
 1. A method of reducing degradation of a solventextraction composition for extracting metals from aqueous solutions,wherein said composition comprises an orthohydroxyaryloxime compound, orsalts, tautomers or metal complexes thereof, the method comprisingadding to the solvent extraction composition, and/or a solventextraction circuit containing said solvent extraction composition, ananti-degradation agent chosen from a member selected from the groupconsisting of: i) a phenol compound substituted with a member selectedfrom the group consisting of 2 t-butyl groups; from 1 to 2 t-butylgroups and a C₁-C₄ alkyl group; from 1 to 3 alkaryl groups; and a cumylgroup; ii) a catechol compound substituted with a solubilizing groupchosen from a member selected from the group consisting of an optionallysubstituted C₆-C₂₀ alkyl; and an optionally substituted C₆-C₂₀ aryl;iii) a resorcinol compound substituted with a solubilizing group chosenfrom a member selected from the group consisting of an optionallysubstituted C₆-C₂₀ alkyl; and an optionally substituted C₆-C₂₀ aryl; andiv) mixtures of (i)-(iii), thereby stabilizing the orthohydroxyaryloximecompound or salts, tautomers or metal complexes thereof, and reducingdegradation of the solvent extraction composition.
 2. A method accordingto claim 1, wherein the orthohydroxyaryloxime compound is chosen from amember selected from the group consisting of a 5-(C₈ to C₁₄alkyl)-2-hydroxyacetophenone oxime; a 5-(C₈ to C₁₄alkyl)-2-hydroxybenzaldoxime; and mixtures thereof.
 3. A methodaccording to claim 2, wherein the orthohydroxyaryloxime compound is amixture of aldoxime and ketoxime, and wherein the weight ratio ofaldoxime:ketoxime is from 90:10 to 50:50.
 4. A method according to claim1, wherein the anti-degradation agent is chosen from a compound selectedfrom the group consisting of


5. A method according to claim 4, wherein the anti-degradation agent ischosen from a member selected from the group consisting of mono-, di-,or tri- (1-phenylethyl) phenol; isomers thereof; and mixtures thereof.6. A method according to claim 1, wherein the anti-degradation agent ispresent in an amount of from 0.1% to 20% by weight of the total weightof the solvent extraction composition.
 7. A method according to claim 6,wherein the anti-degradation agent is present in an amount of from 0.5%to 10% by weight of the total weight of the solvent extractioncomposition.
 8. A method according to claim 1, wherein the solventextraction composition further comprises an equilibrium modifier.
 9. Amethod according to claim 8, wherein the equilibrium modifier is chosenfrom one or more compounds selected from the group consisting of2,2,4-trimethyl-1,3-pentanediol mono-isobutyrate;2,2,4-trimethyl-1,3-pentanediol mono-benzoate;2,2,4-trimethyl-1,3-pentanediol di-isobutyrate;2,2,4-trimethyl-1,3-pentanediol di-benzoate; butyl adipate; pentyladipate; hexyl adipate; isobutyl heptyl ketone; nonanone; diundecylketone; 5,8-diethyldodecane-6,7-dione; tridecanol; and nonyl phenol. 10.A method according to claim 1, wherein the degradation of the solventextraction composition is due to nitration and/or oxidation.